tracing: Add hist trigger support for pausing and continuing a trace

Allow users to append 'pause' or 'continue' to an existing trigger in
order to have it paused or to have a paused trace continue.

This expands the hist trigger syntax from this:
    # echo hist:keys=xxx:vals=yyy:sort=zzz.descending \
          [ if filter] >> event/trigger

to this:

    # echo hist:keys=xxx:vals=yyy:sort=zzz.descending:pause or cont \
          [ if filter] >> event/trigger

Link: http://lkml.kernel.org/r/b672a92c14702cb924cdf6fc27ea1809bed04907.1457029949.git.tom.zanussi@linux.intel.com

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Tested-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -3837,6 +3837,7 @@ static const char readme_msg[] =
 	"\t            [:values=<field1[,field2,...]>]\n"
 	"\t            [:sort=<field1[,field2,...]>]\n"
 	"\t            [:size=#entries]\n"
+	"\t            [:pause][:continue]\n"
 	"\t            [if <filter>]\n\n"
 	"\t    When a matching event is hit, an entry is added to a hash\n"
 	"\t    table using the key(s) and value(s) named, and the value of a\n"
@@ -3851,7 +3852,11 @@ static const char readme_msg[] =
 	"\t    used to specify more or fewer than the default 2048 entries\n"
 	"\t    for the hashtable size.\n\n"
 	"\t    Reading the 'hist' file for the event will dump the hash\n"
-	"\t    table in its entirety to stdout."
+	"\t    table in its entirety to stdout.\n\n"
+	"\t    The 'pause' parameter can be used to pause an existing hist\n"
+	"\t    trigger or to start a hist trigger but not log any events\n"
+	"\t    until told to do so.  'continue' can be used to start or\n"
+	"\t    restart a paused hist trigger.\n\n"
 #endif
 ;
 

kernel/trace/trace_events_hist.c

@@ -86,6 +86,8 @@ struct hist_trigger_attrs {
 	char		*keys_str;
 	char		*vals_str;
 	char		*sort_key_str;
+	bool		pause;
+	bool		cont;
 	unsigned int	map_bits;
 };
 
@@ -193,6 +195,11 @@ static struct hist_trigger_attrs *parse_hist_trigger_attrs(char *trigger_str)
 			attrs->vals_str = kstrdup(str, GFP_KERNEL);
 		else if (strncmp(str, "sort=", strlen("sort=")) == 0)
 			attrs->sort_key_str = kstrdup(str, GFP_KERNEL);
+		else if (strcmp(str, "pause") == 0)
+			attrs->pause = true;
+		else if ((strcmp(str, "cont") == 0) ||
+			 (strcmp(str, "continue") == 0))
+			attrs->cont = true;
 		else if (strncmp(str, "size=", strlen("size=")) == 0) {
 			int map_bits = parse_map_size(str);
 
@@ -871,6 +878,9 @@ static int event_hist_trigger_print(struct seq_file *m,
 	if (data->filter_str)
 		seq_printf(m, " if %s", data->filter_str);
 
+	if (data->paused)
+		seq_puts(m, " [paused]");
+	else
 		seq_puts(m, " [active]");
 
 	seq_putc(m, '\n');
@@ -910,16 +920,30 @@ static int hist_register_trigger(char *glob, struct event_trigger_ops *ops,
 				 struct event_trigger_data *data,
 				 struct trace_event_file *file)
 {
+	struct hist_trigger_data *hist_data = data->private_data;
 	struct event_trigger_data *test;
 	int ret = 0;
 
 	list_for_each_entry_rcu(test, &file->triggers, list) {
 		if (test->cmd_ops->trigger_type == ETT_EVENT_HIST) {
+			if (hist_data->attrs->pause)
+				test->paused = true;
+			else if (hist_data->attrs->cont)
+				test->paused = false;
+			else
 				ret = -EEXIST;
 			goto out;
 		}
 	}
 
+	if (hist_data->attrs->cont) {
+		ret = -ENOENT;
+		goto out;
+	}
+
+	if (hist_data->attrs->pause)
+		data->paused = true;
+
 	if (data->ops->init) {
 		ret = data->ops->init(data->ops, data);
 		if (ret < 0)
@@ -1011,6 +1035,7 @@ static int event_hist_trigger_func(struct event_command *cmd_ops,
 	 * triggers registered a failure too.
 	 */
 	if (!ret) {
+		if (!(attrs->pause || attrs->cont))
 			ret = -ENOENT;
 		goto out_free;
 	} else if (ret < 0)